#sudo apt-get update
update is used to resynchronize the package index files from their sources.
source index file in /etc/apt/ sources.lst
#sudo apt-get upgrade
upgrade is used to install the newest versions of all packages currently installed on the system (include kernel)
#sudo apt-get install package_name
example: sudo apt-get install ssh
install is followed by one or more packages desired for installation.
#sudo apt-get remove package_name
emove is identical to install except that packages are removed instead of installed.